Magnetic acicular alloy particles containing iron as a main component

ABSTRACT

Magnetic acicular alloy particles containing iron as a main component according to the present invention, have an average major axial diameter of 0.05 to 0.2 μm, and pH values of water suspensions of said magnetic acicular alloy particles containing iron as a main component, which satisfies the formula: 
     
       
         ( pH  value of water suspension treated by  A  method)−( pH  value of water suspension treated by  B  method)&lt;0  
       
     
     Such magnetic acicular alloy particles containing iron as a main component exhibit an excellent dispersibility in a vehicle, especially such a vehicle composed of a binder resin having a polar group such as —SO 3 M (wherein M is H, Na or K), —COOH or the like, and in which an orientation and a packing density in coating film are improved.

This application is a division of application Ser. No. 09/361,112, filedJul. 27, 1999, now U.S. Pat. No. 6,447,618,the entire content of whichis hereby incorporated by reference in this application.

BACKGROUND OF THE INVENTION

The present invention relates to magnetic acicular alloy particlescontaining iron as a main component, and more particularly, to magneticacicular alloy particles containing iron as a main component whichexhibit an excellent dispersibility in a vehicle, especially such avehicle composed of a binder resin having a polar functional group suchas —SO₃M (wherein M is H, Na or K), —COOH or the like, and in which anorientation and a packing density in coating film are improved; and amagnetic recording medium using the magnetic acicular alloy particlescontaining iron as a main component.

In recent years, recording-time prolongation, the miniaturization andlightening of magnetic recording and reproducing apparatuses for audioor video have proceeded more rapidly. In particular, VTRs (video taperecorders) are now widespread, so that there have been intenselydeveloped VTRs aiming at prolongation of the recording-time,miniaturization and lightening of the VTRs.

On the other hand, magnetic recording media such as magnetic tapes havebeen required to have a still higher performance and a still higherrecording density.

More specifically, magnetic recording media have been required to show ahigh image quality, high output characteristics, and especially highfrequency characteristics. For this reason, it has been stronglydemanded to enhance an SIN ratio of magnetic recording media.

These characteristics of the magnetic recording media have a closerelation to the magnetic particles used therefor. In recent years,magnetic acicular alloy particles containing iron as a main componenthave been used as magnetic particles for magnetic recording media suchas digital audio tapes (DAT), 8-mm video tapes, Hi-8 tapes, video floppyor the like, because such particles can show a high coercive force and alarge saturation magnetization as compared to those of the conventionalmagnetic iron oxide particles.

In magnetic recording fields, there has been a continuous demand for theenhancement of these characteristics. In particular, it has beenstrongly demanded to further improve properties of the magnetic acicularalloy particles containing iron as a main component, which have aconsiderable influence on the characteristics of magnetic recordingmedia such as S/N ratio or the like.

Namely, it has now been strongly required to provide magnetic acicularalloy particles containing iron as a main component, which show anexcellent dispersibility in vehicle, and in which an orientation and apacking density in coating film are improved, and to improve propertiesof the magnetic acicular alloy particles themselves.

In order to attain the excellent dispersibility in vehicle, and theenhanced orientation and packing density in coating film, there havebeen widely proposed a method of improving surface conditions of themagnetic acicular alloy particles containing iron as a main component byusing various organic or inorganic compounds, or a method of usingbinder resins having a polar functional group such as —SO₃M, —COOH orthe like.

In order to satisfy the requirements for improving the properties of themagnetic acicular alloy particles themselves, there have been conductedvarious attempts not only for reducing the size of the magnetic acicularalloy particles containing iron as a main component, but also forenhancing the shape-maintaining property and the aspect ratio (majoraxial diameter/minor axial diameter) which tend to be deteriorated inassociation with the size reduction. In addition, it has been attemptedto improve magnetic properties of these magnetic acicular alloyparticles containing iron as a main component. In order to obtainmagnetic acicular alloy particles containing iron as a main component,which have the above-mentioned excellent properties, it is necessarythat acicular goethite particles used as a starting material are fineparticles having a large aspect ratio and a narrow particle sizedistribution.

In addition, it is important that in the production of the magneticacicular alloy particles, the particle shape of the raw aciculargoethite particles is retained as similarly as possible. In order toallow the magnetic acicular alloy particles to inherit the particleshape of the raw acicular goethite particles, it has been attempted tocoat the surfaces of the acicular goethite particles or acicularhematite particles obtained by heat-dehydrating the acicular goethiteparticles, with various inorganic or organic compounds in advance of theheat-reduction thereof.

As well known in the arts, the size reduction of the magnetic acicularalloy particles containing iron as a main component adversely affectsthe orientation and packing density in coating film. In addition, as amatter of course, various inorganic compounds coated on the surfaces ofthe acicular goethite particles or the acicular hematite particles stillremains on the surfaces of the magnetic acicular alloy particlescontaining iron as a main component, which are obtained by subjectingthe acicular goethite particles or the acicular hematite particles toheat-reduction, or are diffused thereinto, thereby deteriorating thesurface conditions of the magnetic acicular alloy particles containingiron as a main component. As a result, the conformability orcompatibility of the particles with binder resin is considerablydeteriorated.

Therefore, the magnetic acicular alloy particles containing iron as amain component have been strongly required to show an enhanceddispersibility in vehicle, high orientation and packing density incoating film, without being adversely influenced by the size reductionor the surface conditions thereof.

Hitherto, in order to improve various properties of the magneticacicular alloy particles containing iron as a main component, such asdispersibility in vehicle, oxidation stability or the like, there isknown such a method of contacting the magnetic acicular alloy particlescontaining iron as a main component with a basic gas such as ammonia, atvarious stages of the production process thereof, e.g., at a stageimmediately after the heat-reduction and before allowing any oxide layerto be formed on the surfaces of the magnetic acicular alloy particlescontaining iron as a main component, at a gradual oxidation stage afterthe heat-reduction, or at a stage where the oxide layer is alreadyformed on the surfaces of the particles by gradual oxidation (JapanesePatent Application Laid-Open (KOKAI) Nos. 49-89899(1974),49-99004(1974), 51-51796(1976) and 51-63494(1976), Japanese PatentPublication (KOKOKU) No. 55-4802(1980), Japanese Patent ApplicationLaid-Open (KOKAI) Nos. 61-270315(1986), 62-156202(1987), 63-88806(1988)and 3-101103(1991), Japanese Patent Publication (KOKOKU) No.5-57321(1993), Japanese Patent Application Laid-Open (KOKAI) No.6-29112(1994), etc.).

Thus, at the present time, it has been most strongly demanded to providemagnetic acicular alloy particles containing iron as a main component,which show an excellent dispersibility in vehicle, and in which a highorientation and a high packing density in coating film are improved,without being adversely affected by the size reduction or surfaceconditions thereof. However, the above-mentioned conventional methodsfail to sufficiently satisfy these requirements.

Specifically, in the case of the particles described in Japanese PatentApplication Laid-Open (KOKAI) Nos. 49-89899(1974), 49-99004(1974),51-51796(1976) and 51-63494(1976), Japanese Patent Publication (KOKOKU)No. 55-4802(1980), Japanese Patent Application Laid-Open (KOKAI) Nos.61-270315(1986), 62-156202(1987), 63-88806(1988) and 3-101103(1991),Japanese Patent Publication (KOKOKU) No. 5-57321(1993), Japanese PatentApplication Laid-Open (KOKAI) No. 6-29112(1994), etc., the differencebetween pH values of the water suspensions containing these particles asdescribed hereinafter, is not less than 0. In particular, in the case ofthe particles described in Japanese Patent Application Laid-Open (KOKAI)No. 63-88806(1988), the difference between pH values of the watersuspensions is equal to zero as shown in Comparative Example 7 below,and the dispersibility in vehicle and the orientation and packingdensity in coating film are still unsatisfactory.

As a result of the present inventors' earnest studies, it has been foundthat magnetic acicular alloy particles containing iron as a maincomponent, which show that the difference between pH values of two watersuspensions thereof respectively treated by a boiling method (A method)and an ordinary temperature method (B method) according to JIS K5101-1991, is less than 0 [i.e., (pH value of water suspension treatedby A method)−(pH value of water suspension treated by B method)<0], andwhich have an average major axial diameter of 0.05 to 0.2 μm, can showan excellent dispersibility in vehicle, and can exhibit enhancedorientation and packing density in coating film. The present inventionhas been attained on the basis of the finding.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide magnetic acicularalloy particles containing iron as a main component, which can show anexcellent dispersibility in vehicle, and exhibit enhanced orientationand packing density in coating film, without being adversely influencedby the reduction of the particle size and surface conditions thereof.

To accomplish the aims, in a first aspect of the present invention,there are provided magnetic acicular alloy particles containing iron asa main component, which have an average major axial diameter of 0.05 to0.2 μm, and the difference between pH values of water suspensions of themagnetic acicular alloy particles containing iron as a main component,which is represented by the formula:

(pH value of water suspension treated by A method)−(pH value of watersuspension treated by B method)<0

wherein

the A method comprises:

placing 5 g of the magnetic acicular alloy particles containing iron asa main component in a hard conical flask;

adding 100 ml of water into the flask;

after heating to boiling for 5 minutes, boiling the content of the flaskfor 5 minutes;

compensating a reduced amount of water in the flask by adding theretowater which is previously boiled to remove a carbon dioxide gastherefrom; and

after plugging the flask, allowing the content of the flask to stand forcooling to room temperature, and

the B method comprises:

placing 5 g of the magnetic acicular alloy particles containing iron asa main component in a hard conical flask;

adding 100 ml of water which is previously boiled to remove a carbondioxide gas therefrom, into the flask; and

after plugging the flask, shaking the content of the flask for 5minutes, and

the pH values of the water suspensions respectively treated by the A andB methods are measured by a pH meter.

In a second aspect of the present invention, there is provided amagnetic recording medium comprising:

a non-magnetic substrate; and

a magnetic layer formed on the substrate, comprising a binder resin andthe magnetic acicular alloy particles containing iron as a maincomponent set forth in-the first aspect.

DETAILED DESCRIPTION OF THE INVENTION

First, the magnetic acicular alloy particles containing iron as a maincomponent according to the present invention are explained.

The magnetic acicular alloy particles containing iron as a maincomponent according to the present invention, contain iron in an amountof usually not less than 50% by weight, preferably 50 to 95% by weight,more preferably 60 to 90% by weight based on the weight of theparticles, and may optionally contain at least one of other elementssuch as Al, Co, Ni, P, Si, B, rare earth elements or the like, ifdesired. These optional elements may be contained in an amount ofusually less than 50% by weight, preferably not less than 5% by weightand less than 50% by weight, more preferably 10 to 40% by weight basedon the weight of the particles. Further, in the consideration of goodshape-maintaining property accompanied by the size reduction of theparticles, large aspect ratio (major axial diameter/minor axialdiameter) and high magnetic properties, it is preferred that themagnetic acicular alloy particles containing iron as a main componentare composed of iron and aluminum, cobalt and/or a rare earth element.

The content of Al is preferably 0.1 to 30 mol % based on the totalcontent of metal elements in the magnetic acicular alloy particlescontaining iron as a main component. When the content of Al is less than0.1 mol %, the sintering-prevention effect upon heat-reduction of thestarting particles may be insufficient, so that it may become difficultto inherit the shape of the starting particles. As a result, theobtained magnetic acicular alloy particles containing iron as a maincomponent fail to show the aimed particle shape and, therefore, anappropriate aspect ratio. On the other hand, when the content of Al ismore than 30 mol %, the heat-reduction of the starting particles may beinhibited from proceeding, and it may become difficult to obtainmagnetic acicular alloy particles containing iron as a main component,which have a large saturation magnetization (σs), because of theincrease in amounts of components which do not contribute toimprovements in magnetic properties thereof.

Cobalt is an element suited for enhancing various properties of theobtained particles such as saturation magnetization, oxidationstability, coercive force distribution (Switching Field Distribution:SFD) or the like. The content of Co is preferably 0.5 to 35 mol % basedon the total content of metal elements in the magnetic acicular alloyparticles containing iron as a main component.

The content of rare earth element is preferably 0.1 to 10 mol % based onthe total content of metal elements in the magnetic acicular alloyparticles containing iron as a main component. When the content of rareearth element is less than 0.1 mol %, the sintering-prevention effectupon the heat-reduction of the starting particles may be insufficient,so that it may become difficult to inherit the shape of the startingparticles. As a result, the obtained magnetic acicular alloy particlescontaining iron as a main component fail to show the aimed particleshape and, therefore, an appropriate aspect ratio. On the other hand,when the content of rare earth element is more than 10 mol %, the heatreduction of the starting particles may be inhibited from proceeding,and it may become difficult to obtain magnetic acicular alloy particlescontaining iron as a main component, which have a large saturationmagnetization (σs), because of the increase in amounts of componentswhich do not contribute to improvements in magnetic properties thereof.From the standpoint of industrial applicability, the use of relativelyinexpensive Nd, Y, La and Sm is preferable.

The magnetic acicular alloy particles containing iron as a maincomponent, have an average major axial diameter of usually 0.05 to 0.2μm, preferably 0.08 to 0.18 μm, and an aspect ratio of preferably notless than 3:1, more preferably not less than 6:1. In the considerationof the dispersibility in vehicle, the upper limit of the aspect ratio ispreferably 20:1, more preferably 15:1.

The magnetic acicular alloy particles containing iron as a maincomponent according to the present invention may have an acicular shapeincluding spindle, rice-ball, acicular or the like. However, among them,the spindle-shaped particles which have a uniform particle sizedistribution and are free from the inclusion of dendritic particles, arepreferred. The spindle-shaped particles have a major axial diameterdistribution of preferably not more than 0.4, more preferably not morethan 0.35. The lower limit of the major axial diameter distribution ispreferably 0.1.

The magnetic acicular alloy particles containing iron as a maincomponent according to the present invention can be produced by using asstarting particles, such spindle-shaped particles which have a uniformparticle size distribution and are free from the inclusion of dendriticparticles, and which are obtained by reacting a aqueous ferrous saltsolution with either an aqueous alkali carbonate solution or both anaqueous alkali hydroxide solution and an aqueous alkali carbonatesolution.

As to the magnetic properties of the magnetic acicular alloy particlescontaining iron as a main component according to the present invention,in the consideration of various properties of the obtained magneticrecording media such as high-density recording or the like, the coerciveforce thereof is preferably 1,400 to 2,500 Oe, more preferably 1,500 to2,500 Oe; and the saturation magnetization thereof is preferably 100 to170 emu/g, more preferably 120 to 160 emu/g.

In the magnetic acicular alloy particles containing iron as a maincomponent according to the present invention, it is important that thepH values of water suspensions thereof respectively treated by A and Bmethods as described in the pH-measuring method according to JISK5101-1991, can satisfy such a relationship as represented by theformula:

[pH value of water suspension treated by the A method (boilingmethod)]−[pH value of water suspension treated by the B method (ordinarytemperature method)]<0,

when the pH values are measured by a pH-measuring method according toJIS Z 8802. The difference value of {(pH value of water suspensiontreated by A method)−(pH value of water suspension treated by B method)}(hereinafter referred to as merely “difference between pH values”) ispreferably not more than −0.1, more preferably not more than −0.2. Thelower limit of the difference between pH values is preferably −0.5, morepreferably −0.4.

When the difference between pH values is not less than 0, it is notpossible to obtain the aimed magnetic acicular alloy particlescontaining iron as a main component, which can show an excellentdispersibility in vehicle, and can show enhanced orientation and packingdensity in coating film.

It is preferred that the content of ammoniacal nitrogen in the magneticacicular alloy particles containing iron as a main component accordingto the present invention, is 30 to 800 ppm, more preferably 30 to 500ppm. When the content of ammoniacal nitrogen is less than 30 ppm, the pHvalue of the water suspension treated by the B method (hereinafterreferred to merely as “ordinary temperature method”) may beinsufficiently high, so that the difference between pH values may tendto be not less than 0. On the other hand, when the content of ammoniacalnitrogen is more than 800 ppm, the pH value of the water suspensiontreated by the ordinary temperature method is sufficiently high, so thatthe difference between pH values tends to be less than 0. However, inthis case, the aimed effects of the present invention are alreadysaturated or reduced and, therefore, the use of such a large amount ofammoniacal nitrogen is unnecessary. Incidentally, the term of“ammoniacal nitrogen”-means that measured by an ammonium ion-measuringmethod described in JIS K0102-1993.

It is preferred that the content of soluble salts such as soluble alkalimetal salts such as Na salts, K salts or the like, or soluble alkaliearth metal salts such as Ca salts, Mg salts or the like (hereinafterreferred to merely as “soluble salts”) in the magnetic acicular alloyparticles containing iron as a main component according to the presentinvention, is not more than 800 ppm, when measured with respect to thewater suspension thereof treated by the A method (hereinafter referredto as “boiling method”). When the content of the soluble salts is morethan 800 ppm, the difference between pH values may tend to be not lessthan 0, because the pH value of the water suspension obtained by theboiling method becomes high. Further, when the particles containing anexcess amount of the soluble salts are dispersed in vehicle, the saltsare reacted with resins so as to form a compound, thereby causingdisadvantages such as drop-out or the like. In order to obtain the aimedmagnetic acicular alloy particles containing iron as a main component,it is preferred that the content of the soluble salts is not more than500 ppm, more preferably not more than 400 ppm.

It is preferred that the content of the soluble alkali metal salt suchas Na salts, K salts or the like in the magnetic acicular alloyparticles containing iron as a main component according to the presentinvention, is not more than 500 ppm, when measured with respect to thewater suspension thereof treated by the boiling method. When the contentof the soluble alkali metal salt is more than 500 ppm, the differencebetween pH values may tend to be not less than 0, because the pH valueof the water suspension obtained by the boiling method becomes high.Further, when the particles containing an excess amount of the solublealkali metal salt are dispersed in vehicle, the salt is reacted withresins so as to form a compound, thereby causing disadvantages such asdrop-out or the like. In order to obtain the aimed magnetic acicularalloy particles containing iron as a main component, the content of thesoluble alkali metal salt is more preferably not more than 400 ppm,still more preferably not more than 300 ppm.

It is preferred that the content of the soluble alkali earth metal saltsuch as Ca salts, Mg salts or the like in the magnetic acicular alloyparticles containing iron as a main component according to the presentinvention, is not more than 300 ppm, more preferably not more than 100ppm, still more preferably not more than 80 ppm, when measured withrespect to the water suspension thereof which is obtained by the boilingmethod. When the content of the soluble alkali earth metal salt is morethan 300 ppm, the difference between pH values may tend to be not lessthan 0, because the pH value of the water suspension obtained by theboiling method becomes high. Further, when the particles containing anexcess amount of the soluble alkali earth metal salt is dispersed invehicle, the salt is reacted with resins so as to form a compound,thereby causing disadvantages such as drop-out or the like.

Next, the process for producing the magnetic acicular alloy particlescontaining iron as a main component according to the present invention,is explained.

In general, magnetic acicular alloy particles containing iron as a maincomponent can be produced by heat-reducing acicular goethite particlesobtained by passing an oxygen-containing gas such as air through asuspension containing an iron-containing precipitate which is obtainedby reacting an aqueous ferrous salt solution with either an aqueousalkali hydroxide solution and/or an aqueous alkali carbonate solution,or acicular hematite particles obtained by subjecting the above aciculargoethite particles to filtering-out, washing with water, drying and thenheat-dehydrating, at a temperature of 300 to 700° C., thereby producingmagnetic acicular alloy particles containing iron as a main component;and then gradually oxidizing the obtained magnetic acicular alloyparticles while passing an oxygen-containing non-reducing gastherethrough, thereby forming an oxide layer on the surfaces thereof. Inthe above-mentioned general method for producing the magnetic acicularalloy particles containing iron as a main component, when the aciculargoethite particles or the acicular hematite particles are subjected tovarious treatments such as washing with pure water, etc., so as toremove soluble salts such as soluble alkali metal salts such as Nasalts, K salts or the like, or soluble alkali earth metal salts such asCa salts, Mg salts or the like therefrom for reducing the contents ofimpurities, and further when the obtained particles are treated at atemperature of 60 to 180° C. under a wet non-reducing gas streamcontaining ammonia and oxygen at the gradual oxidation stage after theheat-reduction or any subsequent stage after completion of the gradualoxidation, it is possible to produce the magnetic acicular alloyparticles containing iron as a main component according to the presentinvention. As the non-reducing gases, the use of nitrogen gas ispreferable.

The acicular goethite particles or the acicular hematite particles havebeen ordinarily produced using sodium salts or potassium salts as astarting aqueous alkali hydroxide solution. In particular, most of theseparticles have been industrially produced using an aqueous sodiumhydroxide solution as the aqueous alkali hydroxide solution, an aqueoussodium carbonate solution as the aqueous alkali carbonate solution, orthe like. In this case, sodium salts derived from the aqueous alkalisolution, or sodium salts derived from Na₂SO₃, etc., as by-product saltsproduced by the reaction between the aqueous ferrous salt solution andthe aqueous alkali solution, are inevitably contained or retained withinthe obtained particles, on the surfaces thereof or between mutuallyentangled particles,

Also, when a potassium salt is used as the starting aqueous alkalisolution, the potassium salt is contained or retained within theparticles, on the surfaces thereof or between mutually entangledparticles. Most of these alkali metal salts such as Na salts or K saltscan be removed by washing the acicular goethite particles produced fromthe aqueous solution, with water under ordinary conditions. However, thealkali metal salts which are still retained within the particles orbetween firmly entangled aggregate particles, cannot be readily removedonly by washing with water. In such a case, the alkali metal salts suchas Na salts or K salts are usually contained or retained in an amount of600 to 2,000 ppm. These alkali metal salts incapable of being removedonly by washing with water, are solubilized at a subsequentheat-dehydration step or heat-reduction step. For this reason, theobtained magnetic acicular alloy particles containing iron as a maincomponent, tend to contain a large amount of soluble alkali metal salts.

Also, the acicular goethite particles or the acicular hematite particlestend to contain or retain alkali earth metal salts such as Ca salts orMg salts resulting from impurities in the aqueous ferrous salt solutionand water as starting materials, or in wash water, in an amount ofusually 200 to 10,000 ppm. These alkali earth metal salts are alsosolubilized at a subsequent heat-dehydration step or heat-reductionstep. For this reason, the obtained magnetic acicular alloy particlescontaining iron as a main component, tend to contain a large amount ofsoluble alkali earth metal salts.

The magnetic acicular alloy particles containing iron as a maincomponent, which have a low soluble salt content, can be obtained, forexample, by a method (i) of producing acicular goethite particles usingstarting materials having a low content of impurities, or a method (ii)of heat-dehydrating acicular goethite particles especially at atemperature of 300 to 800° C. and then washing the thus-obtainedhematite particles with pure water, or the like. From the industrial andeconomical viewpoints, the method (ii) is more advantageous.

The introduction of ammoniacal nitrogen into the magnetic acicular alloyparticles containing iron as a main component according to the presentinvention, can be accomplished by either a method of immersing themagnetic acicular alloy particles containing iron as a main component onthe surfaces of which an oxide layer is formed, in ammonia and thendrying the particles (immersion method), or a method of contacting themagnetic acicular alloy particles containing iron as a main componentwith an ammonia gas in a vapor phase (vapor-phase contact method). Inthe immersion method, water is used as a solvent, so that the coerciveforce and the saturation magnetization of the obtained particles maytend to be deteriorated under certain treating conditions. Therefore,the use of the vapor-phase contact method is more preferred.

In the case of the vapor-phase contact method, it is preferred that theammonia gas be contacted with the particles in the course of the gradualoxidation after heat-reduction (i.e., from initiation to completion ofthe gradual oxidation). In particular, it is more preferable to use anon-reducing gas such as a nitrogen gas which contains water vapor at aconcentration of not less than 0.1%, together with an oxygen gas and anammonia gas.

Next, the magnetic recording medium using the magnetic acicular alloyparticles containing iron as a main component according to the presentinvention, and the process for producing the magnetic recording medium,are explained.

The magnetic recording medium according to the present invention,comprises a non-magnetic substrate; a non-magnetic undercoat layeroptionally formed on the non-magnetic substrate, which is obtained byapplying thereon a non-magnetic coating material composed ofnon-magnetic particles, a binder resin and a solvent, and then dryingthe coat; and a magnetic recording layer formed on the surface of thenon-magnetic substrate or the non-magnetic undercoat layer which isobtained by applying thereon a magnetic coating material composed of themagnetic acicular alloy particles containing iron as a main component, abinder resin and a solvent, and then drying the coat.

As the non-magnetic substrates, there may be exemplified those currentlyordinarily used for magnetic recording media, e.g., synthetic resinfilms such as polyethylene terephthalate film, polyethylene film,polypropylene film, polycarbonates film, polyethylene naphthalate film,polyamides film, polyamide imides film, polyimides film or the like;metal foils or plates such as aluminum, stainless steel or the like; orvarious papers.

As to the blending ratio of the magnetic acicular alloy particlescontaining iron as a main component to the binder resin in the magneticrecording layer, the amount of the magnetic acicular alloy particlesblended is usually 200 to 2,000 parts by weight, preferably 300 to 1,500parts by weight based on 100 parts by weight of the binder resin.

The magnetic recording layer may further contain ordinarily usedadditives such as lubricants, abrasives, anti-static agents or the like.

As the binder resins, there may be exemplified those currentlyordinarily used for the production of magnetic recording media such asvinyl chloride-vinyl acetate copolymers, urethane resins, vinylchloride-vinyl acetate-maleic acid copolymers, urethane elastomers,butadiene-acrylonitrile copolymers, polyvinyl butyral, cellulosederivatives such as nitrocellulose, polyester resins; syntheticrubber-based resins such as polybutadiene, epoxy resins, polyamideresins, polyisocyanate, electron radiation-curable acrylic urethaneresins; a mixture thereof; or the like. These binder resins may containpolar functional groups such as —COOH, —SO₃M or the like. In particular,when the binder resins used contain —COOH or —SO₃M as polar functionalgroups, it is possible to efficiently exhibit the aimed effects of thepresent invention, i.e., the dispersibility in vehicle, and theorientation and packing density in coating film can be remarkablyimproved.

Since the magnetic recording medium using the magnetic acicular alloyparticles containing iron as a main component according to the presentinvention, has a gloss of usually not less than 160%, preferably notless than 170%, more preferably not less than 180%, it is recognizedthat the magnetic acicular alloy particles can show an excellentdispersibility in vehicle.

In addition, since the magnetic recording medium has a squareness ofusually not less than 0.87, preferably not less than 0.88, it isrecognized that the magnetic acicular alloy particles can show anenhanced orientation in coating film.

Further, since the magnetic recording medium has a remanence fluxdensity (Br) of usually not less than 2,800 Gauss, preferably not lessthan 2,900 Gauss, more preferably not less than 3,000 Gauss, it isrecognized that the magnetic acicular alloy particles can show anenhanced packing density in coating film.

The important point of the present invention lies in such a fact thatwhen the difference between pH values of water suspensions containingthe magnetic acicular alloy particles according to the presentinvention, which are respectively obtained by the above A and B methods,is not less than 0, the magnetic acicular alloy particles aredeteriorated in dispersibility in vehicle, and cannot be improved inorientation and packing density in coating film.

As a result of studies concerning factors which have any influences onthe difference between pH values, there has been obtained such aknowledge that the difference between the pH values is considerablyinfluenced by the contents of ammoniacal nitrogen and soluble salts inthe magnetic acicular alloy particles containing iron as a maincomponent. Namely, when the content of ammoniacal nitrogen in themagnetic acicular alloy particles containing iron as a main component is30 to 800 ppm and the content of the soluble salts such as the solublealkali metal salt or the soluble alkali earth metal salt is not morethan a specific amount, the difference between the pH values becomesless than 0. On the other hand, when the content of ammoniacal nitrogenin the magnetic acicular alloy particles containing iron as a maincomponent is out of the above-mentioned range and the content of thesoluble salts such as the soluble alkali metal salt or the solublealkali earth metal salt is more than a specific amount, the differencebetween the pH values becomes not less than 0.

The reason why the difference between the pH values is varied inaccordance with the contents of ammoniacal nitrogen and these solublesalts, is considered as follows, though not clearly known yet. That is,the pH value of the water suspension obtained by the boiling method ismainly attributed to only the amount of the soluble salts such as thesoluble alkali metal salt, the soluble alkali earth metal salt or thelike, because the ammoniacal nitrogen is evaporated and scattered out.On the other hand, the pH value of the water suspension obtained by theordinary temperature method is attributed to the amount of theammoniacal nitrogen as well as that of the soluble salts such as thesoluble alkali metal salt, the soluble alkali earth metal salt or thelike, because the ammoniacal nitrogen is difficult to evaporate and,therefore, still remains therein. Accordingly, by adjusting the contentsof both the ammoniacal nitrogen and the soluble salts to appropriateranges such that the difference between pH values is less than 0, it ispossible to obtain the aimed magnetic acicular alloy particlescontaining iron as a main component according to the present invention,which have various enhanced properties.

Specifically, as shown in Comparative Examples described hereinafter,when the content of the soluble sodium salt is too large even though thecontent of the ammoniacal nitrogen in the magnetic acicular alloyparticles containing iron as a main component falls within the specifiedrange, the pH value of the water suspension treated by the boilingmethod is equal to or larger than that treated by the ordinarytemperature method, so that the difference between the pH values becomesnot less than 0. In addition, when the content of the ammoniacalnitrogen does not fall in the specified range even though the content ofthe soluble salts is small, the pH value of the water suspension treatedby the boiling method is larger than that treated by the ordinarytemperature method, so that the difference between the pH values alsobecomes not less than 0. Therefore, these magnetic acicular alloyparticles containing iron as a main component cannot exhibit the aimedeffects, i.e., various properties thereof cannot be enhanced.

The magnetic acicular alloy particles containing iron as a maincomponent according to the present invention are excellent indispersibility in vehicle and are improved in orientation and packingdensity in coating film. Therefore, the magnetic acicular alloyparticles containing iron as a main component according to the presentinvention can be suitably used as high-performance and high-densityrecording magnetic particles.

Further, the magnetic recording medium using the magnetic acicular alloyparticles containing iron as a main component according to the presentinvention, is not only excellent in gloss, but also can exhibit a highorientation degree and a large saturation flux density (Bm). Therefore,the magnetic recording medium according to the present invention can besuitably used as a high-performance and high-density recording magneticrecording medium.

EXAMPLES

The present invention is described in more detail by Examples andComparative Examples, but the Examples are only illustrative and,therefore, not intended to limit the scope of the present invention.

Various properties were evaluated by the following methods.

(1) The average major axial diameter and average minor axial diameter ofparticles are respectively expressed by average values of major axialdiameters and minor axial diameters of 300 to 350 particles which weresampled from a photograph obtained by expanding an electron micrograph(x 30,000) four times in each of the longitudinal and transversedirections.

(2) The aspect ratio is expressed by a ratio of the average major axialdiameter to the average minor axial diameter.

(3) The major axial diameter distribution is expressed by the ratio of astandard deviation to the average major axis diameter.

The standard deviation was obtained by the following equation:$s = \sqrt{\sum\limits_{i = 1}^{n}{\left( {x_{1} - \overset{\_}{x}} \right)^{2}/n}}$

wherein x₁, x₂, x_(n) represent the determined major axis diameter ofthe each specimen, {overscore (x)} represents an average major axisdiameter determined of the each specimen.

(4) The pH values of water suspensions obtained by treating magneticacicular alloy particles containing iron as a main component by aboiling method and an ordinary temperature method, are expressed by thevalues measured by a method described in JIS K 5101-1991. Namely, the pHvalue of each water suspension obtained by treating 5 g of sampleparticles by the below-described A or B method, was measured accordingto JIS Z 8802 (pH-measuring method §7.).

A method: 100 ml of water was added to a hard conical flask in whichsample particles were placed, and after heating to boiling forapproximately 5 minutes, the contents of the flask were boiling for 5minutes. Water which was previously boiled to remove a carbon dioxidegas therefrom, was added to the flask to compensate a reduced amount(boiling loss) of water. The conical flask was plugged, and the contentsof the flask were cooled to room temperature.

B method: 100 ml of water which was previously boiled to remove a carbondioxide gas therefrom, was added to a hard conical flask in which sampleparticles were placed, and the flask was plugged and then shaken for 5minutes, thereby mixing the contents thereof together.

(5) The total content of Na, K, Ca, Mg, Fe, Al, Co and rare earthelements in the particles, is expressed by the value measured asfollows. That is, 0.2 g of sample particles were immersed in a 25%aqueous hydrochloric acid solution, and then the mixture was heated andboiled to dissolve the particles therein, thereby obtaining a solution.After cooling the solution to ordinary temperature, pure water was addedthereto to prepare 100 cc of a solution. The thus obtained solution wasmeasured using an inductively coupled high-frequency plasma atomicemission spectroscope (SPF-400 Model, manufactured by Seiko Denshi KogyoCo., Ltd.), thereby obtaining the above total content. The content ofeach element in soluble salts of Na, K, Ca and Mg is expressed by thevalue obtained by measuring a water suspension treated by the aboveboiling method or ordinary temperature method using an inductivelycoupled high-frequency plasma atomic emission spectroscope (SPF-400Model, manufactured by Seiko Denshi Kogyo Co., Ltd.). The content of Siin the particles is expressed by the value measured according to“General Rule of Fluorescent X-Ray Analysis” of JIS K 0119 using afluorescent X-ray diffractometer (3063 M-Model, manufactured by RigakuDenki Kogyo Co., Ltd.).

(6) The content of ammoniacal nitrogen in magnetic acicular alloyparticles containing iron as a main component is expressed by the valuemeasured by an ammonium ion [NH4⁺] measuring method described in JIS K0102-1993, §42. That is, the sample particles were pretreated by amethod described in JIS K 0102, §42.1 (distillation method) to separateinterfering substances therefrom, and then the amount of ammonium ionwas measured by an indophenol blue absorptiometric method described inJIS K 0102-1993, §42.2.

(7) The magnetic properties of magnetic acicular alloy particlescontaining iron as a main component, is expressed by the value measuredusing “Vibration Sample-type Magnetometer VSM-3S-15 (manufactured byToei Kogyo Co., Ltd.) by applying thereto an external magnetic field ofup to 10 kOe.

(8) As to the gloss on the surface of coating film, the 45° glossthereon was measured by a glossmeter UGV-5D (manufactured by SugaTesting Machines Mfg. Co., Ltd.).

Example 1

<Production of Magnetic Spindle Alloy Particles Containing Iron as aMain Component>

First, spindle-shaped goethite particles obtained by washing with purewater and having an average major axial diameter of 0.25 μm, an aspectratio (major axial diameter/minor axial diameter) of 13:1 and a majoraxial diameter distribution of 0.21 (Al content: 8.2 mol %, Co content:3.5 mol % and Nd content: 1.4 mol %, based on the total content of metalelements (Fe, Al, Co and Nd); and Na content: 950 ppm, Ca content: 151ppm, Mg content: 135 ppm, K content: 42 ppm based on the total amount ofthe spindle-shaped goethite particles) were prepared. The spindle-shapedgoethite particles were heat-dehydrated in air at 750° C., therebyobtaining spindle-shaped hematite particles. The thus-obtainedspindle-shaped hematite particles had an Na content of 1030 ppm, a Cacontent of 159 ppm, an Mg content of 158 ppm and a K content of 47 ppm.The spindle-shaped hematite particles were pulverized, deaggregated andwashed with pure water, and then successively subjected to filtering,molding and drying. The thus-treated spindle-shaped hematite particleshad an Na content of 101 ppm, a Ca content of 155 ppm, an Mg content of152 ppm and a K content of 16 ppm.

100 g of the obtained spindle-shaped hematite particles having a reducedsoluble salt content, were subjected to heat-reduction at a temperatureof 450° C. under a hydrogen gas stream until the dew point thereofreached −40° C., thereby producing magnetic spindle-shaped alloyparticles containing iron as a main component, which contain Al, Co andNd. After completion of the heat-reduction, the hydrogen gas stream wasreplaced with a nitrogen gas stream, and then the magneticspindle-shaped alloy particles were cooled to 70° C.

Next, while controlling the gas temperature at 70° C., a mixed gashaving an oxygen gas concentration of 0.450% (air) and a water-vaporconcentration of 0.367% based on 35 liters of the nitrogen gas, waspassed through the magnetic spindle-shaped alloy particles, therebyforming an oxide layer on the surfaces of the particles. Further, anammonia gas whose concentration in the mixed gas was adjusted to 0.024%,was introduced and contacted with the magnetic spindle-shaped alloyparticles.

When the temperature of the magnetic spindle-shaped alloy particles wasincreased to 145° C. at which the heat-generation was terminated, thegas temperature was cooled to room temperature during which the supplyof the mixed gas was continued.

The thus obtained magnetic spindle-shaped alloy particles containingiron as a main component, which contain Al, Co and Nd, had an averagemajor axial diameter of 0.18 μm, an aspect ratio of 8:1, a major axialdiameter distribution of 0.28, a coercive force of 1,590 Oe and asaturation magnetization value of 131 emu/g. The pH value of themagnetic spindle-shaped alloy particles was 9.8 when treated by theabove boiling method (A method), and was 10 when treated by the ordinarytemperature method (B method). As a result, it was confirmed that thedifference between the pH values was −0.2. Further, the magneticspindle-shaped alloy particles containing iron as a main component,which contain Al, Co and Nd, had an Al content of 8.0 mol %, a Cocontent of 3.4 mol % and an Nd content of 1.3 mol % based on the totalcontent of metal elements (Fe, Al, Co and Nd) in the particles, andfurther had an Na content of 145 ppm (soluble Na content: 143 ppm in thecase of the boiling method, and 110 ppm in the case of the ordinarytemperature method), a K content of 22 ppm (soluble K content: 20 ppm inthe case of the boiling method, and 19 ppm in the case of the ordinarytemperature method), a Ca content of 202 ppm (soluble Ca content: 65 ppmin the case of the boiling method, and 8 ppm in the case of the ordinarytemperature method) and an Mg content of 196 ppm (soluble Mg content: 5ppm in the case of the boiling method, and 3 ppm in the case of theordinary temperature method) based on the magnetic spindle-shaped alloyparticles containing iron as a main component, which contain Al, Co andNd. The total amount of the soluble salts was 233 ppm. The ammoniacalnitrogen content of the magnetic spindle-shaped alloy particles was 290ppm.

Example 2

<Production of Magnetic Recording Medium>

12 g of the magnetic spindle-shaped alloy particles containing iron as amain component, which were obtained in Example 1, 1.2 g of an abrasive(tradename: AKP-50, produced by Sumitomo Chemical Co., Ltd.), 0.24 g ofcarbon black (tradename: #2400B, produced by Mitsubishi Chemical Corp.),a binder resin solution (composed of 30% by weight of a vinylchloride-vinyl acetate copolymer resin having a sodium sulfonate groupand 70% by weight of cyclohexanone) and cyclohexanone were mixedtogether. The obtained mixture (solid content: 78%) was further kneadedby a plastomill for 30 minutes, thereby obtaining a kneaded material.

The thus obtained kneaded material was charged into a 140-ml glassbottle together with 95 g of 1.5 mmφ glass beads, a binder resinsolution (composed of 30% by weight of a polyurethane resin having asodium sulfonate group and 70% by weight of a mixed solvent (methylethyl ketone:toluene=1:1)), cyclohexanone, methyl ethyl ketone andtoluene. The mixture was mixed and dispersed together for 6 hours usinga paint shaker. Further, a lubricant and a curing agent were added tothe resultant mixture, and the obtained mixture was mixed and dispersedtogether for 15 minutes using a paint shaker, thereby obtaining amagnetic coating material.

The thus obtained magnetic coating material had the followingcomposition:

Magnetic spindle-shaped alloy 100 parts by weight particles containingiron as a main component Vinyl chloride-vinyl acetate 10 parts by weightcopolymer resin having a sodium sulfonate group Polyurethane resinhaving a 10 parts by weight sodium sulfonate group Abrasive (AKP-30) 10parts by weight Carbon black (#3250B) 2.0 parts by weight Lubricant(myristic acid:butyl 3.0 parts by weight stearate = 1:2) Curing agent(polyisocyanate) 5.0 parts by weight Cyclohexanone 65.3 parts by weightMethyl ethyl ketone 163.3 parts by weight Toluene 98.0 parts by weight

The magnetic coating material was applied on a 12 μm-thick polyethyleneterephthalate film using an applicator to form a coating layer having athickness of 15 μm. Thereafter, the film was oriented and dried in themagnetic field, and then subjected to calender treatment. Then, thecoating layer was subjected to curing reaction at 60° C. for 24 hoursand the film was slit into 5-inch width, thereby obtaining a magnetictape. The thickness of the obtained magnetic layer was 1.0 μm.

The obtained magnetic tape had a gloss of 200%, a squareness (Br/Bm) of0.900, a remanence flux density value (Br) of 3,120 Gauss and a coerciveforce value (Hc) of 1,520 Oe.

Examples 3 to 9 and Comparative Examples 1 to 7

The same procedure as defined in Example 1 was conducted except thatvarious starting materials having different compositions, soluble alkalimetal salt content, soluble alkali earth metal salt content and solublesalt contents, were used, and conditions of heat-reduction, gradualoxidation and the like were varied, thereby obtaining various magneticacicular alloy particles containing iron as a main component havingdifferent compositions, soluble alkali metal salt content, solublealkali earth metal salt content and soluble salt contents.

The thus obtained magnetic acicular alloy particles containing iron as amain component are shown in Tables 1 and 2.

Incidentally, the magnetic acicular alloy particles containing iron as amain component which were obtained in Comparative Example 6, wereparticles produced in the same manner as in Example 1 except that thespindle-shaped hematite particles were immediately subjected toreduction reaction without washing with pure water; after replacinghydrogen gas with ammonia gas, the obtained particles were treated withammonia gas at 250° C. for one hour; and then the obtained particleswere gradually oxidized with a mixed gas composed of nitrogen and oxygen(air) when the temperature of the particles reached 70° C. As shown inTable 2, the magnetic acicular alloy particles containing iron as a maincomponent which were obtained in Comparative Example 6, had anammoniacal nitrogen content of 0 ppm. As a result, it was recognizedthat in the case where the particles were contacted with the ammonia gasat a temperature as high as 250° C., the ammoniacal nitrogen could notbe effectively introduced into the particles.

In addition, the magnetic acicular alloy particles containing iron as amain component which were obtained in Comparative Example 7, wereparticles produced in the same manner as in Example 1 except that thespindle-shaped hematite particles were immediately subjected toreduction reaction without washing with pure water; the obtainedparticles were gradually oxidized with a mixed gas composed of nitrogenand oxygen (air) when the temperature of the particles reached 70° C.;the particles were heated to 100° C. under a nitrogen gas stream andtreated with a mixed gas composed of ammonia gas and nitrogen for onehour; and then the obtained particles were further heated to 250° C.under a nitrogen stream and treated with the mixed gas composed ofammonia gas and nitrogen for one hour (i.e., the method described inJapanese Patent Application Laid-Open (KOKAI) No. 63-88806(1989)). Asshown in Table 2, the magnetic acicular alloy particles containing ironas a main component which were obtained in Comparative Example 7, had anammoniacal nitrogen content of 25 ppm. As a result, it was alsorecognized that in the case where the particles were contacted with theammonia gas and then treated under a nitrogen stream, the ammoniacalnitrogen could not effectively introduced into the particles.

Examples 10 to 18 and Comparative Examples 8 to 15

The same procedure as defined in Example 2 was conducted except thatkinds of magnetic acicular alloy particles containing iron as a maincomponent and kinds of functional groups of binder resins were varied,thereby obtaining magnetic recording media.

Various properties of the obtained magnetic recording media are shown inTable 3.

TABLE 1 Magnetic acicular alloy particles containing iron as a maincomponent Composition (mol %) based on Fe, Al, Co, Examples rare earthand Si and Rare earth Other Comparative Al Co element element Examples(mol %) (mol %) (mol %) (mol %) Example 3 8.1 3.4 Nd: 2.2 — Example 48.0 3.4 La: 2.2 — Example 5 5.4 7.9 Nd: 4.5 — Example 6 10.6 26.3 Y: 7.1— Example 7 10.6 4.4 — — Example 8 — 2.6 Nd: 2.7 Si: 2.7 Example 9 8.03.5 Nd: 1.3 — Comparative 8.0 3.4 Nd: 2.2 — Example 1 Comparative 8.13.4 Nd: 2.2 — Example 2 Comparative 8.1 3.4 Nd: 2.2 — Example 3Comparative 8.1 3.4 Nd: 2.1 — Example 4 Comparative 8.0 3.4 Nd: 2.2 —Example 5 Comparative 8.0 3.4 Nd: 2.2 — Example 6 Comparative 8.0 3.4Nd: 2.2 — Example 7 Examples Magnetic acicular alloy particles andcontaining iron as a main component Comparative Average major axialExamples diameter (μm) Aspect ratio Example 3 0.17 10:1  Example 4 0.159:1 Example 5 0.11 8:1 Example 6 0.09 6:1 Example 7 0.18 10:1  Example 80.19 9:1 Example 9 0.18 9:1 Comparative 0.17 10:1  Example 1 Comparative0.15 8:1 Example 2 Comparative 0.18 8:1 Example 3 Comparative 0.17 10:1 Example 4 Comparative 0.17 10:1  Example 5 Comparative 0.18 7:1 Example6 Comparative 0.17 8:1 Example 7 Examples Magnetic acicular alloyparticles and containing iron as a main component Comparative Majoraxial diameter Examples distribution Particle shape Example 3 0.23Spindle-shaped Example 4 0.23 Spindle-shaped Example 5 0.22Spindle-shaped Example 6 0.21 Spindle-shaped Example 7 0.29Spindle-shaped Example 8 0.34 Spindle-shaped Example 9 0.48 AcicularComparative 0.28 Spindle-shaped Example 1 Comparative 0.33Spindle-shaped Example 2 Comparative 0.37 Spindle-shaped Example 3Comparative 0.33 Spindle-shaped Example 4 Comparative 0.34Spindle-shaped Example 5 Comparative 0.45 Spindle-shaped Example 6Comparative 0.36 Spindle-shaped Example 7

TABLE 2 Magnetic acicular alloy particles containing iron as a maincomponent Examples pH and Ordinary Difference Comparative Boilingtemperature between pH Examples method method values Example 3 9.8 10.1−0.3 Example 4 10.0 10.2 −0.2 Example 5 9.8 10.1 −0.3 Example 6 9.8 10.0−0.2 Example 7 10.2 10.7 −0.5 Example 8 9.1 9.2 −0.1 Example 9 10.5 10.3−0.2 Comparative 9.7 9.6 +0.1 Example 1 Comparative 10.3 10.0 +0.3Example 2 Comparative 10.8 10.3 +0.5 Example 3 Comparative 10.3 10.1+0.2 Example 4 Comparative 10.5 10.4 +0.1 Example 5 Comparative 10.310.2 +0.1 Example 6 Comparative 10.1 10.1 0.0 Example 7 Magneticacicular alloy particles containing iron as a main component Solublealkali metal salt Soluble Na Soluble K Examples Ordinary Ordinary andtempera- tempera- Comparative Boiling ture Boiling ture Examples methodmethod method method Example 3 146 112 21 19 Example 4 258 190 15 13Example 5 87 69 8 5 Example 6 318 240 2 2 Example 7 390 300 23 19Example 8 120 91 19 16 Example 9 100 75 21 19 Comparative 154 109 19 17Example 1 Comparative 750 589 43 36 Example 2 Comparative 1,500 1,235 5849 Example 3 Comparative 900 786 31 25 Example 4 Comparative 765 621 2 1Example 5 Comparative 1,589 1,301 67 61 Example 6 Comparative 1,5271,258 59 44 Example 7 Magnetic acicular alloy particles containing ironas a main component Soluble alkali earth metal salt Soluble Ca SolubleMg Examples Ordinary Ordinary and tempera- tempera- Comparative Boilingture Boiling ture Examples method method method method Example 3 6 1 4 2Example 4 13 2 5 2 Example 5 78 18 3 1 Example 6 40 7 2 1 Example 7 9029 6 3 Example 8 12 2 5 3 Example 9 70 7 4 3 Comparative 6 2 4 2 Example1 Comparative 290 56 396 197 Example 2 Comparative 1,687 369 123 52Example 3 Comparative 40 3 122 46 Example 4 Comparative 50 4 300 161Example 5 Comparative 71 7 5 2 Example 6 Comparative 64 5 4 2 Example 7Magnetic acicular alloy particles containing iron as a main componentTotal Satura- Examples content tion and of Ammoniacal Coercive magnet-Comparative soluble nitrogen force Hc ization Examples salts (ppm) (Oe)(emu/g) Example 3 177 300 1,680 135 Example 4 291 165 1,850 128 Example5 176 405 1,920 142 Example 6 362 90 2,200 139 Example 7 509 755 1,550126 Example 8 156 205 1,710 131 Example 9 195 260 1,780 134 Comparative183 0 1,690 134 Example 1 Comparative 1,479 0 1,800 129 Example 2Comparative 3,368 0 1,570 133 Example 3 Comparative 1,093 310 1,660 135Example 4 Comparative 1,117 1,500 1,650 134 Example 5 Comparative 1,7320 880 101 Example 6 Comparative 1,654 25 1,510 122 Example 7

TABLE 3 Examples Magnetic recording medium and Kind of resin Kind ofComparative (functional magnetic Examples group) particles Gloss (%)Example 10 —SO₃Na Example 3 202 Example 11 —SO₃Na Example 4 200 Example12 —SO₃Na Example 5 220 Example 13 —SO₃Na Example 6 207 Example 14—SO₃Na Example 7 188 Example 15 —SO₃Na Example 8 177 Example 16 —COOHExample 3 196 Example 17 —OH Example 3 180 Example 18 —SO₃Na Example 9172 Comparative —SO₃Na Comparative 150 Example 8 Example 1 Comparative—SO₃Na Comparative 140 Example 9 Example 2 Comparative —SO₃NaComparative 128 Example 10 Example 3 Comparative —SO₃Na Comparative 140Example 11 Example 4 Comparative —SO₃Na Comparative 147 Example 12Example 5 Comparative —SO₃Na Comparative 118 Example 13 Example 6Comparative —SO₃Na Comparative 148 Example 14 Example 7 Comparative —OHComparative 141 Example 15 Example 1 Examples Magnetic recording mediumand Remanence Coercive Comparative flux density force Hc ExamplesSquareness (Br) (Gauss) (Oe) Example 10 0.915 3,200 1,620 Example 110.908 3,210 1,840 Example 12 0.918 3,280 2,050 Example 13 0.907 3,3002,310 Example 14 0.883 2,920 1,480 Example 15 0.870 3,150 1,690 Example16 0.900 3,210 1,600 Example 17 0.879 2,990 1,580 Example 18 0.875 3,1501,640 Comparative 0.859 2,650 1,610 Example 8 Comparative 0.844 2,3901,800 Example 9 Comparative 0.837 2,420 1,510 Example 10 Comparative0.853 2,570 1,600 Example 11 Comparative 0.853 2,520 1,600 Example 12Comparative 0.800 1,800   710 Example 13 Comparative 0.820 2,740 1,410Example 14 Comparative 0.851 2,500 1,590 Example 15

What is claimed is:
 1. A magnetic recording medium comprising: anon-magnetic substrate; and a magnetic layer formed on said substrate,comprising a binder resin and magnetic acicular alloy particlescontaining iron as a main component which have an average major axialdiameter of 0.05 to 0.2 μm, an ammoniacal nitrogen content of 30 to 800ppm based on said magnetic acicular alloy particles containing iron as amain component, soluble salts of not more than 800 ppm based on saidmagnetic acicular alloy particles containing iron as a and pH values ofwater suspensions of said magnetic acicular alloy particles containingiron as a main component, which satisfies the formula: (pH value ofwater suspension treated by A method)−(pH value of water suspensiontreated by B method)<0 wherein said A method comprises: placing 5 g ofsaid magnetic acicular alloy particles in a hard conical flask; adding100 ml of water into the flask; after heating to boiling forapproximately 5 minutes, boiling the content of the flask for 5 minutes;compensating a reduced amount of water in the flask by adding theretowater which is previously boiled to remove a carbon dioxide gastherefrom; and after plugging the flask, allowing the content of theflask to stand for cooling to room temperature, said B method comprises:placing 5 g of said magnetic acicular alloy particles in a hard conicalflask; adding 100 ml of water which is previously boiled to remove acarbon dioxide gas therefrom, into the flask; and after plugging theflask, shaking the content of the flask for 5 minutes, and the pH valuesof the respective water suspensions treated by said A and B methods aremeasured by a pH meter.
 2. A magnetic recording medium according toclaim 1, which have a gloss of not less than 160%, a squareness of notless than 0.87 and a remanence flux density of not less than 2,800Gauss.